Bacteriophages are the most abundant biological entities on earth and the selective pressures imposed by these pervasive predators have a profound impact on the composition and the behavior of microbial communities in every ecological setting. The human body is a compilation of complex ecosystems that rely on bacteria. In fact, the number of bacteria living in a healthy human is estimated to be ten times greater than the number of human cells and phage infections that perturb these microbial communities have recently been implicated in a wide range of human disorders. Furthermore, phages are major purveyors of genes that confer virulence and antibiotic resistance and thus phages play a major role in the evolution of bacterial pathogenesis. Clustered regularly interspaced short palindromic repeats (CRISPR) are essential components of a recently discovered, nucleic-acid-based adaptive immune system that is widespread in bacteria and archaea, and these immune systems play a central role in controlling the horizontal dissemination of virulence associated genes. The long-term goal of our research is to understand the impact of CRISPR-mediated immune systems on the evolution and ecology of human associated microbial communities. Specifically, the work outlined in this proposal is aimed at explicating the mechanisms of CRISPR RNA-guided surveillance and targeted elimination of foreign DNA by the adaptive immune system in Escherichia coli. We anticipate that this work will result in high-resolution structures of the CRISPR RNA-guided surveillance complex from Escherichia coli and that these structures will be invaluable to our understanding of how these surveillance machines work. However, structures will only capture snap-shots of the complex in specific poses. To understand the dynamics of these machines we complement our structural studies with real-time kinetic analysis aimed at probing the mechanism and rates of target surveillance. Finally, we implement our structural and biochemical insights to design programmable gene silencing systems. CRISPR-mediated gene silencing offers a novel approach for systematically controlling gene expression in both prokaryotic and eukaryotic systems. Overall, this proposal provides promising new insight to the public and scientific community regarding the mechanisms of adaptive immunity in bacteria that will have significant impacts in biotechnology and medicine.